Air intakes are arranged to supply air to buildings, and/or to machinery or an industrial process of a plant. Normally, the source of the air supply is the ambient, outside air. However, in many location the outside air contains particles, such as for example dust, pollen, mould, salt particles or the like, which may be harmful or unwanted in the supplied air. Therefore, air intakes are in many applications provided with a particle filter for removing such solid unwanted particles from an air stream passing through the air intake.
As an example, air intakes for gas turbine plants are usually provided with one or more particle filters for removing particles from an air or gas flow such that particles are hindered to enter the turbine. With this, potential damage risks such as for example turbine fouling (adhesion of the particles to the rotor blades in the gas turbine air compressing device), corrosion, and erosion (particles that wear on the rotor blades) may be impeded. Thus, filters are provided to ensure continuous operation of the gas turbine plant so that a continuous electricity generation output from the gas turbine can be maintained.
After a certain time of operation, dependent on the ambient conditions in which the gas turbine operates, a large quantity of particles is accumulated in the filters. As a consequence, the pressure drop over the filter (the filter resistance) increases, causing a rise in energy consumption for driving the air stream, a drop in the flow rate of the air stream and/or, a drop in output from the gas turbine plant. Possibly, particles may leek from the filters and follow the air stream into the gas turbine plant. Such a particle intrusion may lead to a deteriorated gas turbine operation, costly maintenance, and/or possibly, a gas turbine breakdown. In order to repair the gas turbine, it may be necessary to shut down the gas turbine plant, which results in an electricity output loss and associated losses in standstill of equipments driven thereby. Therefore, the filters in such air intakes for a plant, for example for a gas turbine plant, are regularly removed and replaced by fresh filters.
However, a filter replacement may raise several problems. If the filter is removed from an intake device during operation of the plant, harmful particles have free passage into the plant causing the above described problems. On the other hand, shut down of the plant is undesired due to complicated process for stopping and start-up, as well as the economical loss associated with standstill.
In the prior art, a rough pre-filter is sometimes mounted before and in series with a high efficiency main filter for protection and prolongation of the life time of the main filter, or in other words, prolongation of the time until a it is necessary to replace the main filter due to an unacceptable rise in pressure drop or an unacceptable risk of particle leakage. However, in some applications, the life time of the main filter and the time interval for main filter exchange is still experienced as too short.